Development of a Thermal Stresses Based Fragmentation Model for Pulverized Coal Particles Gasification by Low Temperature Air Thermal Plasma

Development of a Thermal Stresses Based Fragmentation Model for Pulverized Coal Particles Gasification by Low Temperature

Air Thermal Plasma

25th May 2016ANSYS Convergence Regional Conferences

City Hotel, Ljubljana, Slovenia

Belgrade University, Institute of Nuclear Sciences “Vinca” , Laboratory for Thermal Engineering and Energy, Belgrade,

Serbia

Rastko Jovanović, Dejan Cvetinović, Predrag Stefanović, Predrag Škobalj, Zoran Marković

IntroductionMotivation Model descriptionResults Conclusions Future work

Presentation outline

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Introduction, Plasma fire support technology

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Air-coal mixture channel

Plasma generator Boiler

furnace

Hot air plasma produced in plasma generators is introduced into air – coal mixture duct.The plasma flame with high thermal energy induces coal gasification and partial char oxidation producing highly reactive mixtureThis highly combustible mixture is easily ignited at the furnace entry ensuring high flame stability and overall increased combustion efficiency.

Introduction, Plasma coal gasification process

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Specific phenomena have to be taken into account:.Very high temperaturesComplex reaction mechanisms and kineticsFragmentation

Motivation

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It is necessary to demonstrate its advantages over conventional systemsThe main challenge is that the most CFD codes are suitable for p. f. combustion simulation under conventional conditions.Particle fragmentation is commonly neglected.During plasma coal gasification, very high plasma temperature induces strong particle thermal stresses.These stresses lead to “thermal shock” and extensive particle fragmentation.Fragmentation intensifies devolatilisation (3-4 times) and significantly accelerates char oxidation.The main aims of this work are: development of fragmentation model based on calculations of the thermal stresses inside pulverized coal particles Model implementation in ANSYS FLUENT combustion model using User Defined Functions (UDFs).

Mathematical model, general features

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The reactive flow field was described in Eulerian manner.The turbulence was modeled using the standard k − ε turbulence model together with standard wall functions.Radiation was modeled using Discrete Ordinates (DO) model.The gas radiation absorption coefficient is calculated as function of characteristic cell-size and gas concentrations.Gaseous reactions were modeled using finite rate/eddy dissipation model.Char reactions were modeled using kinetic rate/diffusion limited model.The pulverized coal particles combustion is modeled in a Lagrangian reference frame.

Mathematical model, fragmentation modeling

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Physicalparticles with

same properties

Computationalcell

Strength = 6

Strength = 4

Computationalparticles − parcels

In the Lagrangian approach number of computational particles are chosen to represent actual physical particles with a same characteristics.In order to take into account number of physical particles in a single parcel additional variable termed “strength” is used.


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Standard model − zero dimensional model

TP

rP

vP

Tgas

radiationconvection


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UDF based model − one dimensional model

rP,0 rP,max

i i+1

i-1

rP,i-1

TP,i-1

rP,i+1

TP,i+

1

rP,i

TP,i

Tgas

radiationconvection

Mathematical model, implementation

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Governing transport equations

DPM equations

User FV code -> TP[i]

inert heatingdevolatilization +

fragmentation(Dp,new, mP,new, ρP)

char combustion

σ1,3[i] ≥ σuu/N

σr[i], σt[i] = f(TP[i]) -> σ1[i], σ2[i], σ3[i]

inert heatingdevolatilization

char combustion

yes no

Results, temperature distribution

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No fragmentation


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5 fragments


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8 fragments


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10 fragments

Results, volatile species mass fraction

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10 fragments

No fragmentation

Results, CO species mass fraction

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10 fragments

No fragmentation

Results, volatile and char conversion

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Fragmentation model based on fracture mechanics was developed and implemented into ANSYS FLUENT solver with extensive use of UDFsModel is able to predict crack time and location of crack initialization inside combusting coal particleModel was successfully applied to plasma supported coal particles gasification The obtained results show high influence of “thermal shock” phenomenon on plasma gasification performanceModel will be further developed using trial and error procedure, testing different kinetic constants, particle thermal and transport properties, different particle sizes and different fragmentation criteriaIn the final stage it is expected to compare numerically predicted data with experimental data on pilot plasma burner which is under modernization in Laboratory for Thermal Engineering and Energy, Vinca Institute of Nuclear Sciences, Serbia

Conclusions and future work

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THANK YOU FOR YOUR ATTENTION

Acknowledgments

The authors would like to acknowledge high appreciation for the support and promotion of this work to the Public Enterprise ”Electric power industry of Serbia”, Belgrade, Serbia, and Ministry of Education and Science of Republic of Serbia (Project No. III42010 and TR33050)